1. Field of the Invention
The present invention relates to a method of printing a conductive ink such as an ink comprising a solder paste.
2. Description of the Related Art
To meet the increasing demand for smaller, slimmer, and lighter electronic devices, integrated circuits in chip-size packages (CSPs) have become prevalent. When an integrated circuit of this type is mounted on a printed circuit board, it is electrically coupled to the circuit board through conductive bumps such as solder bumps formed as part of the chip-size package. The chip-size package and solder bumps may be formed during the wafer processing stage, when the integrated circuit is still part of a semiconductor wafer that has not yet been diced into individual chips.
Solder bumps can be formed on a semiconductor wafer by screen printing methods similar to the methods used to form solder bumps on printed circuit boards. One known screen printing method, described in Japanese Patent Application Publication No. 2002-307653, for example, uses air pressure to separate the solder bumps from the stencil through which they are printed.
The screen printer employed in this known method has a vertically movable stage on which the circuit board is mounted, and a frame that holds the stencil. The stage is raised to bring the circuit board into contact with the stencil. Solder paste is forced through the stencil onto the circuit board by a squeegee, forming a pattern of solder bumps. A walled cover is then lowered onto the stencil to create an airtight enclosure into which compressed air is introduced through a tube. Next, the stage is lowered to separate the circuit board from the stencil. The elevated air pressure inside the enclosure pushes the solder bumps out from the stencil, allowing the circuit board and printed solder bumps to separate cleanly from the stencil.
When this method is used to print solder bumps on a semiconductor wafer, it has been found that as the stage is lowered, the downward pull caused by friction between the solder paste and the stencil causes the stencil to warp. The peripheral parts of the wafer therefore separate from the stencil before the central part separates. When the peripheral parts of the wafer separate from the stencil, air escapes through the holes near the periphery of the stencil and the air pressure inside the enclosure falls to the ambient level. With air pressure no longer acting on the solder paste printed in the central part of the wafer, there is a tendency for the solder paste to remain stuck in the central part of the stencil, leading to defects such as missing or inadequately formed solder bumps.